1. Field of the Invention
The present invention relates to film transparency projectors and, more specifically, relates to such projectors having integral means adapted to resist undesired exposure of the film transparency to ultraviolet and infrared radiation originating with the projector's illumination source.
2. Description of the Prior Art
The term "film transparency projector" as used herein shall mean a projector adapted to project an image contained on a film transparency and shall include, but not be limited to motion picture projectors, slide projectors, microfiche and other images made visible to the human eye through illumination.
One of the problems encountered with display of color film transparencies such as slides or motion pictures by means of the use of a projector has been the fact that exposure to ultraviolet or infrared radiation can have a damaging, fading effect on the film. The problem is particularly acute in respect of exposure to ultraviolet radiation which can cause permanent fading of the image on the film transparency.
It has been known to employ a foraminous plate composed of a high thermal conductivity material such as a copper screen to reduce heating of a motion picture during display. See U.S. Pat. No. 1,302,802.
U.S. Pat. No. 1,957,279 discloses illumination of infrared radiation from the radiation of a light source by passing the same through a transparent or transluscent sheet or layer containing a suitable compound for selective absorption. In one embodiment a reflecting layer is combined with a heat absorbing layer.
U.S. Pat. No. 2,464,887 discloses a heat screen for use in projectors wherein the heat screen is relatively thin, of meniscus shape and composed of an alumina-phosphate type of glass. The meniscus shape is said to resist breakage due to thermal expansion and contraction as compared with flat heat screen lenses. A heat absorbing glass is disclosed in U.S. Pat. No. 3,944,352.
U.S. Pat. No. 2,552,184, while alluding to the need to control ultraviolet radiation, teaches no solution to the problem and emphasizes what is said to be the more important problem of controlling infrared radiation. In general, infrared radiation is controlled by employing an interference mirror type of filter which permits passage of the infrared radiation therethrough and reflects the remainder of the visible spectrum. It also acknowledges the teaching of transmitting visible light and reflecting infrared radiation.
There remains a very real and substantial need for a film transparency projector which will effectively resist fading of the film transparency resulting from ultraviolet and infrared radiation.